Clean the mushrooms, wash them briefly and cut into thin slices. Peel onions and cut into thin rings. Wash and chop the pepperoni. Wash parsley, shake dry, pluck and roughly chop.
Cut meat into thin slices and season with salt and pepper. Heat the oil in a frying pan. Fry the meat in it for 1-2 minutes on each side. Remove from the pan and set aside.
Add butter to the pan. Cook the onions and mushrooms for 2-3 minutes. Add chicken stock, crème fraîche and mustard and bring to the boil. Add meat and pepperoni and heat up briefly. Season to taste with salt, pepper and 1 pinch of sugar.
Garnish with parsley.